1. Field of the Invention
The invention pertains to drag reduction in the operation of aircraft and airships by boundary control propulsion and boundary layer control propulsion systems. More particularly, the invention relates to control and flight operation of airship movement using a novel propulsion system having a cluster or array of micro propellers, moveable riblets or a combination of micropropellers and riblets operating within the boundary layer of air around the airship fuselage when in flight. The invention includes a boundary layer propulsion system having micro propellers, riblets or a combination of micro propellers and moveable riblets composed of electroactive polymer (EAP) materials mounted on the airship fuselage in the area of boundary air layer separation to not only reduce drag but also provide maneuvering control over the three flight axis of pitch, yaw and roll of an airship.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98.
The basic concept of a propeller can be dated back to the time of Archimedes (287-212 BC), whose work on ship propulsion has earned him the credit for the invention of the screw propeller. Modern propellers still behave in a manner analogous to rotating a screw or auger through a solid, just as is in Archimedes' time. The thin twisted blades of the modern propellers resemble an airfoil far more than they resemble a screw. An airfoil generates lift by producing a pressure imbalance when air moves around a propeller. Rotating propeller airfoils generate pressure imbalance through the relative movement of air over the curvature of the rotating blade surfaces. A helicopter propeller works the same way as that of an aircraft propeller, the only difference being that the helicopter propeller rotates around a nearly vertical axis whereas an aircraft propeller rotates around the longitudinal axis.
Propeller design has been largely based on the theory of optimum propeller as developed by Betz, Prandtl and Glauert. According to theory, when designing a propeller driven aircraft only a few initial parameters need to be specified when consider the propeller and propulsion system which has generally omitted considerations of boundary layer control and flight control of the aircraft using boundary layer modifications. Instead the prior art has focused upon the optimum propeller design as being a function of: 1) the diameter of the propeller, 2) the axial velocity of the flow, 3) the number of blades, 4) the selected distribution of propeller blade lift and drag coefficients along the radius, 5) the desired thrust of the available shaft power, and 6) the density of the fluid medium. Of all these parameters, the diameter of the propeller has been considered in the prior art to the greatest individual impact on the performance of the propeller.
As a result the largest propeller with the most slender blade is considered to be the most efficient, which is the length by the rotational speed of the propeller to prevent the tip of the propeller from exceeding the speed of sound to avoid energy consuming sonic shock formation. A larger propeller captures more incoming fluid or air and distributes its power and thrust on a larger fluid volume, creating a small pressure imbalance, which is compensated by having a larger area for the pressure imbalance. The small pressure imbalance is a direct consequence of the relative axial velocity (the slippage velocity between the blade surface and the air). The lower the relative slippage velocity, the more aerodynamically efficient is the propeller.
Also known in the prior art is the use of variable pitch control of the propeller in larger and more sophisticated aircraft. Variable pitch control of the propeller blade provides rotational control over the longitudinal axis of the propeller at the propeller hub to provide increased lift on takeoff and reduce drag in flight and accommodate variation in the density of air at different flight attitudes. Variable pitch propellers are located in the same locations as ordinary propellers and hence do not involve boundary control propulsion or boundary layer control for modifying the pitch, roll and yaw flight axis of flight. Variable pitch propellers also have not been made of electroactive polymer (EAP) materials to control the length or specific shape of the propeller itself.
In the prior art various propeller design principles promote the use of larger and slimmer propeller blades. It is impractical to place a propeller having large blades near the boundary layer flow near the surface of an aircraft fuselage. The advantage of operating a propeller well inside the boundary layer is that the air flow inside the turbulent boundary layer tends to have much smaller air speed relative to that of the free streaming air around it from the perspective of the aircraft. A physically equivalent view is to view it from the perspective of the standing air where the boundary layer air acquires forward momentum as it travels close to the frame of the aircraft.
Theoretically, a boundary layer propeller can recapture some of the momentum “stolen” by the turbulent boundary layer air that is responsible for the parasitic drag experienced by the aircraft which increases with speed at a given altitude. However, at low altitudes, i.e. below the stratosphere, the boundary layer surrounding the frame of an aircraft is typically very small at the fore of the body, and gradually increasing toward the aft of the body for a fully streamlined body. This gradual thickening of the boundary layer is typically followed by an abrupt fluid separation right after it passes the mid-plane, where a portion of the boundary layer air flow splits off from the body frame with consequent vortex formation and turbulence. Since the boundary layer only begins to rapidly broaden after the aforementioned fluid separation, and since there would have no advantage to operate the propeller within the turbulent wake, a single large propeller quite simply can not take advantage of the additional proportion force carried by the forward-momentum rich boundary layer air.
One area of boundary layer control recognized in the prior art is the use of boundary layer control to improve fuel efficiency of the aircraft. Boundary Layer Control, BLC, is a generic term used to describe various methods used to reduce the skin friction drag by controlling the turbulent transition, the development of full turbulent flows, and the fluid separation. Among those is boundary layer suction, which is currently being used on aircraft wings to prevent laminar and turbulent fluid separation by removing the innermost sub-layer of the boundary layer to reduce the boundary layer thickness. One method utilizes a suction pump to such boundary layer air from closely spaced transversal slots. Since both laminar-to-turbulent transition and fluid flow separation require boundary layer of a certain thickness, this method is effective in the laboratory where it was shown that fully laminar flow is possible even for Reynolds numbers far exceeding such transition thresholds. However, the development of a boundary layer suction system is complicated by considerations of optimum slot placement, structural modifications, power systems, and amount of suction needed.
Examples of prior art boundary layer suction systems are described in Stewart, et al. GB 479598A, Thwaites, et al GB 6106222A and Anxionnaz U.S. Pat. No. 3,951,360. There are indications also that very little actual gain in power efficiency is possible as the reduction in skin friction is largely balanced by the large suction needed to maintain the laminar flow.
Another popular method is tangential slot injection. This is exactly the opposite of the suction method in that a high-speed air is injected through a backward pointing slot. One prior art example is Mayer, Jr. U.S. Pat. No. 3,779,199. The sudden nozzle acceleration as a result of the pushing of the high-speed jet on the slower moving boundary layer flow can delay the onset of boundary layer flow separation. Again here the gain in the drag reduction must be weighted against the added power needed for pumping the ducted air to the tangential slots at high-speed. A very similar method is boundary layer blowing, which is primarily used to provide temperature control of high temperature components. Wall cooling has also been proposed for the same purpose and for skin friction reduction by damping the Tollmien-Schlichtling instability to delay the laminar-to-turbulent transition. Here again the amount of cooling power may make such drag force saving impractical.
Yet another popular method is passive surface modification. Examples of such passive surface modification include Mabel GB 881570A which employs a resilient coating, Herbert, et al GB 1019359A which utilizes embossing ridges and Battelle Development Corp. GB 1281899A which utilize a plurality of concave regions. The most popular being the application of flow-aligned miniature ribs, or riblets, to control the growth of small eddies in the near wall boundary structure of the layer.
Both dolphins and sharks have natural riblet-like skins which seem to enhance their ability to swim fast. An average drag reduction of about 6% has been widely reported even though some report as high as an 11% reduction. As there is no additional power required, which is in sharp contrast to the above mentioned suction or jet injected method or skin cooling, riblet-based approaches seem to offer the best chance of providing real-world drag reduction. Riblets are simple to apply too, since companies such as 3M have manufactured riblet films with riblet heights ranging from 20 microns to 100 microns for lower altitude flights. Higher riblets would increase the wetted area significantly, thereby increasing the laminar skin drag. Riblets are also relatively insensitive to an adverse Bernoulli pressure gradient. However, the drawback of using non moveable riblets is that when the flow is no longer aligned with the longitudinal direction of the riblets, the performance of riblets deteriorates rapidly.
Lastly, diffusers have been used in confined air flow as that within a ducted propeller to reduce the drag. The diffuser increases the coupling between the turbulent channel flow with the slower moving (in the frame of reference of the channel wall) boundary layer flow. The diffuser is only useful for channeled flow which is diverging since a diverging channel flow is aerodynamically highly unstable as the air is flowing against the Bernoulli pressure gradient. Diffuser offers no substantial value to external or open fluid flow.
None of the aforementioned methods has yet been proven to be capable of reducing the actual propulsion power in a significant way with the possible exceptions of tangential slot injection and the riblet modified skin. The former is capable of significantly reducing the overall propulsion power requirement even through a large, inefficient internal pump is needed to inject the high-speed air jet because of its ability to postpone the onset of flow separation. A separated boundary layer air flow can create a large stagnant vortex wake that drastically increases the pressure drag in a direct proportion to the cross-sectional area of the wake. Riblets are capable of delaying the onset of turbulent boundary layer flow at the expense of smaller increases in the wetted area, and therefore the Blasius skin drag.
Since the onset of turbulent boundary layer vortices is greatly accelerated by the presence of adverse Bernoulli pressure gradient, and since the working of a propeller can be described by the disk actuation theory (momentum theory), which explains the action in terms of the generation of a pressure discontinuity, it would be possible for propellers working close to the boundary layers (more specifically, to be inside the buffer layer and the viscous sub-layer) to, in effect, reverse the pressure gradient. This is substantially similar to the action of a tangentially injected air jet through a slot in that both can provide a local reversal of adverse pressure gradient. However, tangential slot injection also introduces unwanted additional air into the boundary layer, which offsets much of its benefits.
Fluid flow separation can also be understood in terms of the combined effects of adverse pressure gradients and viscosity. Removal of the adverse pressure gradients can move the separation point further downstream, or eliminate its occurrence altogether. Again this can be accomplished by the introduction of propulsion means within the boundary layer to accelerate the boundary layer fluid, leading to the reattachment of the fluid flow, and thinning the boundary layer in the process.
Some Applications of the above principles to aircraft include Platzer U.S. Pat. No. 5,975,462 which provides for a flapping foil propulsion system for reducing drag. Platzer U.S. Pat. No. 5,975,462 describes many of the boundary layer devices previously discussed and indicates that “the use of small propellers would pose an extremely complicated mechanical installation problem.” Other prior art employing a cluster or array of propellers such as Hughey Pub. No. U.S. 2006/0266881 A1 pertains to a vertical takeoff and landing aircraft using a redundant array of propellers. This prior art does not involve boundary layer control BLC for propulsion or control of the various flight axis of pitch, yaw and roll.
Boundary layer control BLC has also been applied to lighter than air aircraft. For example, Whitnah U.S. Pat. No. 3,079,106 utilizes at least one constricted area in the envelope covered by a porous material to provide an air pressure differential across the porous material to reduce drag. Similarly, Sonstegaard U.S. Pat. No. 3,488,019 utilizes a fine shield that allows preferential leakage with a bow and amidships suction and stem blowing and placement of the ballast tanks along streamlines to reduce drag. These boundary layer control devices applied to lighter than air aircraft all use a standard propeller for propelling the airship along with BLC. These prior art airships do not use an array or cluster of micro propellers disposed in the boundary layer. Further such prior art does not employ an array or cluster of micro propellers to control pitch, yaw and roll characteristics of the airship utilizing boundary layer control.
It thus follows that boundary layer propulsion (surface propulsion) can simultaneously provide propulsion force, stabilize the growth of the boundary layer, and suppress or delay the onset of turbulence as well as fluid flow separation. Boundary layer propulsion also steepens the velocity gradient considerably within the near wall, thereby increasing the viscous drag through an increase of the shear stress adjacent to the wall. Boundary layer control can also be used to control the flight path of an aircraft or an airship in the roll pitch and yaw axis of flight more efficiently than conventional control surfaces of aircraft which operate by increasing drag.